part of mesh not rendering
Other programs might have &quot;Backface Culling&quot; enabled by default, which means the faces become invisible / transparent when viewed from the inside or backside of the face (i.e. it's normal is pointing away from the camera).
To correct this you can press Shift+N as suggested in the comment, but you have to be in Edit Mode for that. You can select single faces you want to recalculate - if you don't know which faces are flipped it's better to hit A to select all faces before recalculating. The Recalculate Normals function also has a tickbox &quot;Inside&quot;, which usually makes the inside pointing outwards but can be helpful if Blender miscalculates the normals on a more complex mesh.
You can also press Alt+N for more options of manipulating the face normals like &quot;Flip&quot; for example, to simply flip the direction no matter if Blender thinks it's outside or inside.
If you clearly want to see which faces are flipped before you export a mesh, you can do this by enabling Face Orientation in the overlays. With this option Blender shows the outside of faces colored in blue and the inside or backside of faces colored in red. In my screenshot you can see some red faces, this indicates their normals are flipped. The color overlay is shown in Object Mode and Edit Mode.